Overcurrent protective device



Sept 4 1934 R. R; PITTMAN ,972,357

OVERCURRENT PROTECTIVE DEVICE Filed June l5, 1933 Patented Sept. 4, 1934 v i1 NIT-ao cruel-ca OVERCURRENT PROTECTIVE DEVICE Ralph Pittman, *Pine "Blui, -Ark.

Application June ll5, 1933,-Serial1No.;v675,944

l3=Glaims. Vr(Gl. 'p200-127) vThis invention .relates `.generally 5to sdevices Iing,;it Ayviilibe seenthat .the ihollow tube `10,-of adapted for disconnecting angelectric'circuitffrom vsome suitable insulatingfmaterialaas hcrnzber, a source of energy ,upon ithe passage of afpre- `isrprovidedrat theupper end withthe-metal elec- V determined current through the circuitandz.par trode v12,the latterbeingfrigidly secured tothe *f6 Yticularly to that typexofffusible devicesicommonly upper-endlof the1tube'.lOby.-means of cooperatreferred to as expulsion-fuses. ying threads onthe tube 10:and.the electrode12.

I amv aware that manytypesrof expulsiony fuses :An-integral portionvof the electrode 12 extends are in general use at this time, -but,.=so far aas upwardly 1to-form fthefcontact surfaces 18, Athe .Ig have been able to discoverallofitheseidevices latterbeing ladaptedtoiengage contact clips of 11i) comprise a hollow tube of insulating @material 'anyfsuitableform 'An-openingof suitable size and vmeans forming an arc within thetubegthe `for-:the-passagetherethrough-of-a'fusible element material of Ythe tube usually being adapted .rfor i114 is. provided at'thetopofethe'electrode 12,:this volatilizing upon the occurrence .orianarczthere- .portionbeing also. provided-with afthreadedfend in. It has been-found that such-,a devicefhas to permitthe -securingotthe:removable cap 13 T15 a denite-minimum current'value `below'ffwhich by meansof cooperatingithreads onltheflatter. the device Willrnotproperlyoperate to extinguish -ffnet-electrode 12 is iprovi'ded with -apluralityf'of the arc Within the tube, this `value `depending -inwardlyextending portions 211arrangedzto form upon theproximity of theiarcpathzto.therbounda centrally positioned recess, and an insulating ing surfaces, and the ,material used ,in.;the con element 211, which "mayibe rahorn jber rod or 420 struction of the tube. AIt;has also :been found :tuba-having closed., ends, extends upwardlyinto that such a device has a denitemaximum'value the recess, and-.is thereby centrally supported in above which the tube wallsrare fruptured, this coaxial Vrelationfto thetube 10 atitszupperlend. value'depending particularly uponztheacrossec- `The-spaceftbetween"the inwardlyextending portional area of the opening, ,or"arcx.path,:.through tions -21 permits-aacommunicatingpassageffrom f12`5 the tube. .thefupperendfof the electrode r1.2.to the .annu- The objectoi the `present .improvement iszto larspacefbetweenthe vtube .10-and the insulatprovide a device which will yoperateiproperlyiover ing element 11.

a much greater range of current .Values ,than :The .llowerf electrode '16 rfof the devicefmay be that of previous devices of this :general type, constructed-:of any-suitable metal, an'disrigidly l-3"`3`0 by providing means formaintainingethearcpath secured .adjacent itorthe Ilower y-end of the itube in intimate association with the fboundingsurflby meansof'cooperating threads on the elec- 5 faces, but atthe same-time providing ample: crosstrodeiI. an'dthetubeIO. =In a manner-similar to sectional area through the Harc: pathfto prevent that described .inifconnection with thezupper-cap excessive gas pressure. `12,1.the'insulatingelement llis supported Yin co- 3'5 A further object is the provisionzcffa-simpla axial-relation withtheftube 10 atits lower-end 90 effective and relativelyinexpensive devicefor .the byftherrecess -formed byithe inwardly extending purpose stated. Y 'integral-iportionsv22. integral portion of the vrWith the above and other objectsaiin-Viev/which lower. electrode "lfextendsA downwardly to form will appear as the description fproceedsgthisiinthe contact .surfaces 118 fi'for .engaging contact i4() vention resides in the novelifeatures--iof form, clips of 'suitable vfform. .rAlower'fuse terminal 95 construction, arrangement and combination .of vformed by the `4member 19 ,andthe screw20 is parts hereinafter described andfpointed out rigidlylattached inanysuitable mannerftoithe the claims. externalupper portion:V of fthe! electrode 16.V

In the accompanying drawing (lisheet) '-Sinceinfthis.devicegashaslong been'the prac Vv15. Figure 1 isla side elevational view ofwa-fdevice .ticeiin fthe construction `ofexpulsion fuses, the 100 embodying this invention. tube 10 and the insulating-element .11 Lmay be Figure -2 illustrates'another elevational 'fview constructedsof 'insulating `material adapted Vfor of thedevice, butvhere shoWnin-section. volatilizing-.uponthe-foccurrencerofanarcadja- Figure 3 illustrates 1a vsectional @view of :the cent iathereto, the lowerelectrode 1'6 is provided I device, taken from theiplane indicated by :the with the gas vents 17 -to -prevent the rupture line A3--3 of Fig. 2. of the device due --to excessive pressure vwithin Figure 4 illustrates another sectional fviewrof thetubel.

the device, taken from' theplaneiindicated-fzby "In installing Ya r'fuse link -Within the-device,

the line 4-,4 of'Fig; 2. thefcapi13 isi-removedand thelower endvoffthe 55 Referring'nowmoreginparticulantocthe drawfuse vlink 114 7is urged-downwardly through the 7G bounding insulating Walls, Using a'hcllow-tube,

opening in the electrode 12, the communicating passage between the upper end of the electrode 12 and the annular space formed by the tube 10 and the insulating element 11, the abovementioned annular space, and one of the vent openings 17. The upper button of the fuse link then engages the upper end of the electrode 12, and the cap 13 is screwed thereon to contact the button of the fuse link..-The lower end of the fuse link is bent ,upwardly to `engagethe member 19, and is rigidly attached thereto by the screw 20. The fusible element 15 of the fuse link is preferably located near the upper end of the insulating tube 10.

The insulating element 11I may-be of any suitcertain resilience, and which at the Sametime willr evolve gas when placed in intimate -association with an arc. Since `the fusible element .15 oc'- cupies but aV small portion of the annular space between the element 11 and the tube 10, thev arc will tend to be somewhat localized when rst initiated, resultingv in` a certain impact against the portion of the element. 11 adjacent to the fusible element 15. It is desirable therefore to use an element having a certaindegree of re.- silience in order that the crossfsectional area of the arc path may be somewhat controlled by the magnitude of ,the current inthe arc. is entirely possible to suspend the elementv 11 on springs to supply thedesired resiliencalprefer, for reasons of simplicity/fand economy, 'to employ an insulating element having suicient inherent resilience to adjust itself to the conditions, and a good quality of horn fibermeets this requirement.

It will be seen from the above description that the fusible element l5 is positioned in the annular spacedelned'by the outer surface of the .insulating element 1l and the inner suifaceof the tube 10, and that, upon the passage of sufficient current through the device, the fusible element 15 will4 be melted, and the circuit-interrupting i arc established in the above-mentioned annular space.

It has long been recognized that devices which depend upon the generation of gaswithin a chamber bounded byinsulating materialdue to the heat of the arc to therebyextinguish it are most effective when the arc is held in intimate relation with the gas-evolving insulating material. For this reason small bore tubes used in expulsion fuses have long been recognized .as eiectivearc extinguishing devices, but the dini# culty has been that the cross-sectional area of the arc `path in such'av device is so small that excessive pressure, tube erosion and flameA length make them undesirable for heavy, currents.

Recognizingthe principle that,'foraY given cross-sectional area of arc .path lthe effectiveness of arc extinction dependsupon the nearness of the arc to the 4insulating material bounding' the arc path, a practical'exam'ple .of the application of this invention will bedescribed. V

, Assume that a deviceis'requiredwhich is effectivexin extinguishing comparatively low current arcs, and for this reason fno portion'of the arc path mustbe` removed morethan lg/ fromfthe the bore of the tube would be of 1/4 inchdiametei, and the cross-sectional.. area of the' arc path 0.04909 squarefinch. qSuch a device; becauseof the smallbore, may be fairly effective at 'currents up to the order of 20D-300 amperes, 60 cycles, but above this range the gas pressure and tube erosion increase rapidly with increased currents, until finally the tube ruptures.

Using the construction here shown, and meeting the same condition, (i. e. that no portion of the arc path be removed more than 1/8" from the bounding surfaces) the inside diameter of the tube 10 may be 1%? inches, and the outside diameter of the insulating element 11 one inch, thus providing a 1/4 inch annular arc space. The condition of proximity of insulation to the arc path has thus been met, but it will be seen that the cross-sectional area of the arc path is that of the annulusdeiined by the 11/2 inch tube and the 1 inchV insulating element, which in this case is 0.9817 square inch. This area is substantially twentytimes that of the 1A; inch bore tube, or approximately equivalent to a tube having a bore vof 1%; inches.

The device thus constructed has therefore approximately.twenty times the interrupting capacity of a 1A inch bore tube of equivalent wall strength, but at the same time is eiective on the low-:currents above mentioned. A device embcdying the arrangement of parts shown herein has, therefore, arc extinguishing properties not .to be obtained in the tube, and over a much greater range of current values with a minimum gas pressure and greatly reduced wall erosion.

A further advantage of the arrangement shown herein lies in the fact that ample space is provided for the Iiexible conductors 14, which are used to connect the fusible element 15. This is of especial importance in fuse links above 100 amperes rating, as the connecting conductors must be largein cross section to avoid overheating. In the simple tube device, these elements are of such size that they substantially block the egress of the gases, and thus contribute to the rupturing ofthe tube walls. r

y While the above constitutes a description of one embodiment of this invention, and to the best of my knowledge a recital of the manner in which the device functions, it is to be understood that such changes as come 'within the scope of the appended claims may be made without departing from the spirit of the invention. For example, a device according to my invention in which the insulating element 11`is not disposed in exact coaxial relation with the tube 10 would be an effective arc extinguishing structure, although I have herein illustrated and described only the preferred embodiment employing a coaxial arrangement of these elements.

v.'I claim asmy invention:

1. An expulsion fuse comprising a hollow tube .of insulating material, an upper conducting elec-V sulating element, and means including a hemisr pherical' upper end portion of said insulating elem'e'nt engaging 'a complementary surface on the lower end of said upper electrode whereby, upon 'the initiation of a circuit-hiterrupting arcV due to the meltingl of fsaidifusible element, the gaspres sure incident thereto may deect laterally the upper portion 'of said insulating element 'for a distance dependent upon the magnitude of the current in the circuit-interrupting arc.

2. An expulsion fuse comprising a hollow tube of insulating material, conducting electrodes rigidly xed on said tube near the respective ends thereof, an inherently resilient cylindrical element of insulating material having an outside diameter less than the inside diameter of said tube supported by said electrodes within said tube, a detachably-mounted fuse-engaging member supported on each of said electrodes externally of said tube, and a passageway extending through each of said electrodes in communication with the inside of said hollow tube whereby a fuse link may be supported within said tube independent of said insulating element.

3. An expulsion fuse comprising a hollow tubular insulator, conducting electrodes spaced apart on said insulator near the respective ends thereof, a cylindrical element of insulating material having an outside diameter less than the inside diameter of said insulator supported by said electrodes and extending through said insulator, a detachable fuse-engaging member on each of said electrodes mounted externally of said insulator, and a passageway extending through said electrodes in communication with the inside of said hollow insulator whereby a fuse link may be installed in said fuse without removing said electrodes or said insulating element.

RALPH R. PITTMAN. 

